1. Field of the Invention
The invention relates generally to the field of photography, and specifically to a mirror system for a single lens reflex camera. More particularly, the invention relates to a mirror system which is improved to reduce the clearance required for movement of its swingable mirror.
2. Description of the Prior Art
Typically in a single lens reflex camera, a flip-up mirror is located between the objective lens and the film. The mirror is normally disposed at an angle of 45.degree., centered on the optical axis of the objective lens, but it can flip up about a pivot pin at the top of the mirror to lie flat. When the mirror is disposed at the 45.degree. angle, it reflects the rays of light that come through the objective lens, on to a ground glass screen in a viewfinder of the camera. A field lens and a penta-roof prism in the viewfinder direct the light rays through an eyelens at the rear of the camera to enable the subject to be photographed to be viewed through the objective lens. The prism has several reflecting surfaces which turn the image formed by the objective lens on the ground glass screen upright as well as right-way-round. When the mirror is flipped up to lie flat, it covers the underside of the ground glass screen and the light rays from the objective lens fall on the film. The positioning of the mirror is such that when an image of the subject to be photographed is focused on the ground glass screen, with the mirror at the 45.degree. angle, the same image falls into focus on the film when the mirror is flipped out of the way.
In many reflex cameras, the mirror is moved out of the way by a spring which is released immediately before the shutter is actuated to take a picture in response to finger pressure against a shutter release button on the outside of the camera body. The mirror there has to be retensioned before every exposure (and in fact before an image can be observed again on the ground glass screen). On most reflexes this retensioning is coupled with the film transport and/or shutter tensioning. Alternatively, an instant return mirror may be employed. The mirror there flips back immediately after the exposure.
Movement of the flip-up mirror between its 45.degree. position and its flipped-up position requires a fair amount of space. Consequently, the objective lens must have a sufficient back focus to provide ample clearance for the mirror movement. Various systems have been tried to reduce the clearance required. One of the earliest consists of supporting the mirror along its horizontal center axis, and swinging it back and up rather than hinging it at the top. More modern designs involve systems where the mirror drops down into the bottom of the camera housing, or folds up along its middle. Another variation, used in 18.times.24 mm miniature reflexes, is a sideways mirror movement. Since the 18.times.24 mm image is upright in the camera, this movement along the long side of the negative takes up less space.